The sun maintains its brightness because of(A) fission of Helium nucl…

The correct answer is Option (3) → (C) only

The Sun’s energy comes from thermonuclear fusion in its core, where the temperature is about \( 1.5\times10^{7} \) K. In the proton–proton cycle, four hydrogen nuclei fuse into one helium nucleus:

    \( \displaystyle 4\,{}^{1}_{1}\mathrm{H} \rightarrow {}^{4}_{2}\mathrm{He} + 2e^{+} + 2\nu + 26.7\,\mathrm{MeV} \)

(A) fission of Helium nuclei — Incorrect. Helium is a light, tightly bound nucleus; energy from light nuclei comes from fusion, not fission.

(B) fusion of \( {}^{4}\mathrm{He} \) nuclei — Incorrect. Helium is the product of the Sun’s fusion. Helium fusion begins only at a later stage in a star’s life, after the hydrogen in the core is used up.

(C) fusion of Hydrogen nuclei — Correct. This is the proton–proton cycle shown above.

(D) fission of Uranium nuclei — Incorrect. The Sun contains no significant amount of uranium; fission of uranium is the energy source of nuclear reactors.

NCERT (2026-27):
Nuclear Fusion – Energy Generation in Stars – “… Thermonuclear fusion is the source of energy output in the interior of stars. … The fusion reaction in the sun is a multi-step process in which the hydrogen is burned into helium. Thus, the fuel in the sun is the hydrogen in its core. … Thus, four hydrogen atoms combine to form an \( {}^{4}_{2}\mathrm{He} \) atom with a release of 26.7 MeV of energy.”